fix(adt): honor HTTP_PROXY/HTTPS_PROXY for WebSocket connections

[dme007]

Summary

Extends #13 (a1af66f — "honor HTTP_PROXY/HTTPS_PROXY environment variables")
from the HTTP transport to the WebSocket path. On SAP Business Application
Studio destinations (Cloud Connector + Connectivity Proxy) every outbound
connection must go through the proxy; the HTTP-only tools started working
after #13 but every WebSocket-backed feature still failed to connect.

All WebSocket traffic in vsp — ABAP debugger, AMDP debugger, RFC execution,
report execution, abapGit WebSocket export — runs through a single
BaseWebSocketClient in pkg/adt/websocket_base.go. Neither the
websocket.Dialer nor the transient *http.Client used on the 401-retry
pre-auth path set a Proxy, so HTTP_PROXY / HTTPS_PROXY were ignored
for WebSocket connects.

The fix

Two small package-level constructors in websocket_base.go so the dialer
and the pre-auth client are created in a single, testable place:

newZADTVSPDialer(tlsConfig) → websocket.Dialer{
    Proxy:            http.ProxyFromEnvironment,
    HandshakeTimeout: 30 * time.Second,
    TLSClientConfig:  tlsConfig,
}

newPreAuthHTTPClient(jar, tlsConfig) → &http.Client{
    Transport: &http.Transport{
        Proxy:           http.ProxyFromEnvironment,
        TLSClientConfig: tlsConfig,
    },
    Jar:     jar,
    Timeout: ...,
}

The two inline blocks in BaseWebSocketClient.Connect now call these
constructors; dialer.Jar = jar on the 401-retry path stays unchanged.

Behaviour for non-proxy deployments

Unchanged. http.ProxyFromEnvironment returns nil when HTTP_PROXY /
HTTPS_PROXY / NO_PROXY are unset, so the dialer connects directly
as before. Same semantics net/http uses by default.

Regression coverage

TestNewZADTVSPDialer_HonoursProxyEnv       — dialer.Proxy non-nil,
    TLS config and handshake timeout preserved.
TestNewPreAuthHTTPClient_HonoursProxyEnv   — transport.Proxy non-nil,
    supplied jar is used, timeout non-zero.

Mirrors the style of TestNewHTTPClient at pkg/adt/config_test.go:171-192
that locks in the HTTP-side #13 fix.

Files changed

pkg/adt/websocket_base.go       — newZADTVSPDialer + newPreAuthHTTPClient
                                  constructors; Connect uses them
pkg/adt/websocket_base_test.go  — new (the two tests above)

Related issues

• vsp does not honor HTTP(S)_PROXY in SAP Business Application Studio (BAS) #13 — original HTTP-side proxy support. This PR is the WebSocket-side
  equivalent; same motivation, same environment, same semantics.